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Martin J. O'Riordan via llvm-dev

2015-Nov-02 10:27 UTC

[llvm-dev] Questions about load/store incrementing address modes

Thanks Steve, I will try this out.  I hadn’t realised that TableGen was
restricted to matching instructions with more than one output operand.  I’m
assuming that this is only a limitation for inferring an instruction from the
patterns, because it does seem to manage schedules okay.

 

Curiously, my memory Reg32+Reg16 pattern is very similar to yours (the 16-bit
offset is sign-extended though):

 

// Memory address: 32-bit base register + 16-bit offset register

def ADDRrr : ComplexPattern<iPTR, 2, "SelectADDRrr", []>;

def MEMrr : Operand<iPTR> {

  let PrintMethod = "printMemOffsetOperand";

  let MIOperandInfo = (ops RC32, RC16_l);

}

 

but it is still happy to select for offset’s > 16-bits.  There is something I
am just not yet getting right, but it looks like I am on the right track.

 

All the best,

 

            MartinO

 

From: Steve Montgomery [mailto:stephen.montgomery3 at btinternet.com] 
Sent: 30 October 2015 13:47
To: Martin J. O'Riordan
Cc: LLVM Developers
Subject: Re: [llvm-dev] Questions about load/store incrementing address modes

 

I’ve implemented something similar, though maybe not similar enough to be able
to help you. My target supports increment and decrement, both pre and post, by
an explicit constant in the range 1 to 8.

 

I didn’t write any custom lowering code, just used setIndexedLoadAction() and
setIndexedStoreAction() in my ISelLowering to identify the types and actions,
e.g.

 

            setIndexedLoadAction(ISD::PRE_INC, MVT::i16, Legal);

 

I didn’t think it was possible to match an indexed load or store using a
TableGen pattern because it can’t match instructions with more than one output
operand, though maybe things have changed and I’ve not noticed. So, I wrote
custom code in my ISelDAGToDAG to match indexed loads and store and to select
these instructions. That seems to work fine for constant increments/decrements.
I don’t know whether the indexed load and store actions would deal with
increment or decrement by a register.

 

I’ve also got an addressing mode similar to yours in which an address can be
formed from a 16-bit register plus a zero-extended 8-bit register. I used a
ComplexPattern to match the address expression and MIOperandInfo to specify the
classes of the registers, e.g.

 

            def memR16R8 : Operand<i16> {

                        let MIOperandInfo = (ops Reg16Class, Reg8Class);

                        ...

            }

 

Again, this seems to work OK for me.

 

Steve

 

On 30 Oct 2015, at 10:43, Martin J. O'Riordan via llvm-dev <llvm-dev at
lists.llvm.org> wrote:

 

I have a rudimentary implementation for load and store instructions, where the
memory address operand is automatically post-incremented when the load or store
instruction is issued.  However, this is currently coded using custom lowering,
and explicit pattern matching in the ‘ISelDAGtoDAG’ implementation.  But it
seems to me that I ought to be able to achieve this exclusively using TableGen
with minimal custom C++ code, but I can’t quite get the patterns to work to
achieve this.

 

I have two types of post-increment:

 

1.      increment by an implied constant which is the size of the object being
loaded or stored

2.      increment by the value contained in another register

 

It should be possible to do these with the same underlying pattern, but
providing a DAG fragment like ‘(i32 2)’ for an ‘i16’ load explicitly in the
TableGen specialisation for the first form, and the actual register chain for
the second.

 

But I don’t seem to be quite get it to work, and in some cases the patterns I
have attempted have crashed TableGen itself L  The incrementing store sort of
works, but the incrementing load doesn’t (at least not using TableGen alone).

 

 

The other form of addressing mode is where the address is a “base” plus signed
“offset” register.  My patterns for this work okay, but I have a limitation. 
The “offset” register is 16-bits, and in particular, it is the low-order 16-bits
of a 32-bit actual register.  But even though I have explicitly stated in
TableGen that the offset register is 16-bits and stated the appropriate register
class, the code-generation still uses the instructions “as if” the offset
register is 32-bits.

 

Memory addresses in this architecture are 32-bits.

 

Has anybody solved similar problems, and have advice or examples of how to do
this?  Or is simply something that TableGen descriptions cannot completely
describe?  I have looked at the other targets for inspiration, but they don’t
quite seem to do what I need.

 

Thanks,

 

            Martin O’Riordan - Movidius Ltd.

 

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Steve Montgomery via llvm-dev

2015-Nov-02 20:25 UTC

head link

[llvm-dev] Questions about load/store incrementing address modes

> On 2 Nov 2015, at 10:27, Martin J. O'Riordan <martin.oriordan at
movidius.com> wrote:
> 
> Thanks Steve, I will try this out.  I hadn’t realised that TableGen was
restricted to matching instructions with more than one output operand.  I’m
assuming that this is only a limitation for inferring an instruction from the
patterns, because it does seem to manage schedules okay.
I’m basing my statement on the material at the end of the “Selection DAG Select
Phase” in “The LLVM Target-Independent Code Generator”,
http://llvm.org/docs/CodeGenerator.html#selectiondag-select-phase. I’ve not
actually checked TableGen though so can’t be 100% sure that the documentation is
still in date.
 > Curiously, my memory Reg32+Reg16 pattern is very similar to yours (the
16-bit offset is sign-extended though):
>  
> // Memory address: 32-bit base register + 16-bit offset register
> def ADDRrr : ComplexPattern<iPTR, 2, "SelectADDRrr", []>;
> def MEMrr : Operand<iPTR> {
>   let PrintMethod = "printMemOffsetOperand";
>   let MIOperandInfo = (ops RC32, RC16_l);
> }
>  
> but it is still happy to select for offset’s > 16-bits.  There is
something I am just not yet getting right, but it looks like I am on the right
track.
I believe that the MIOperandInfo will constrain the register class for your
16-bit offset operand to RC16_1 but in itself it won’t affect the matching of
the operand. Your SelectADDRrr will need to contain code to match an i32 added
to a sign-extended i16. If you’ve already done that, then I’m out of ideas,
sorry.

Steve
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Martin J. O'Riordan via llvm-dev

2015-Nov-02 21:16 UTC

head link

[llvm-dev] Questions about load/store incrementing address modes

Thanks again for your help Steve,

 

I’m thinking perhaps my “SelectADDRrr” pattern is inadequate.  The
sign-extension is at the hardware level, the code generator sees (should see) it
as a 16-bit signed register value.  My implementation is just:

 

bool SHAVEISelDAGtoDAG::SelectADDRrr(SDValue &Addr, SDValue &Base,
SDValue &Offset) {

  if ((Addr.getOpcode() == ISD::ADD) {

    Base = Addr.getOperand(0);

    Offset = Addr.getOperand(1);

    return true;

  }

 

  return false;

}

 

I don’t have any special checks on the offset (or the base for that matter) on
the naive assumption that it would not have been invoked if the constraints were
not already met.  But don’t worry about it, you’ve given me a fresh avenue to
investigate - a few DEBUG dumps should show me the error of my ways :)  I’m
guessing that I need to check that the offset operand is truly a 16-bit register
and return false if it isn’t.  A nice simple fix if that is all that is needed -
thanks again for shedding light on this for me.

 

            MartinO

 

From: Steve Montgomery [mailto:stephen.montgomery3 at btinternet.com] 
Sent: 02 November 2015 20:25
To: Martin J. O'Riordan <martin.oriordan at movidius.com>
Cc: LLVM Developers <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Questions about load/store incrementing address modes

 

 

On 2 Nov 2015, at 10:27, Martin J. O'Riordan <martin.oriordan at
movidius.com <mailto:martin.oriordan at movidius.com> > wrote:

 

Thanks Steve, I will try this out.  I hadn’t realised that TableGen was
restricted to matching instructions with more than one output operand.  I’m
assuming that this is only a limitation for inferring an instruction from the
patterns, because it does seem to manage schedules okay.

 

I’m basing my statement on the material at the end of the “Selection DAG Select
Phase” in “The LLVM Target-Independent Code Generator”,
http://llvm.org/docs/CodeGenerator.html#selectiondag-select-phase. I’ve not
actually checked TableGen though so can’t be 100% sure that the documentation is
still in date.

 



Curiously, my memory Reg32+Reg16 pattern is very similar to yours (the 16-bit
offset is sign-extended though):

 

// Memory address: 32-bit base register + 16-bit offset register

def ADDRrr : ComplexPattern<iPTR, 2, "SelectADDRrr", []>;

def MEMrr : Operand<iPTR> {

  let PrintMethod = "printMemOffsetOperand";

  let MIOperandInfo = (ops RC32, RC16_l);

}

 

but it is still happy to select for offset’s > 16-bits.  There is something I
am just not yet getting right, but it looks like I am on the right track.

 

I believe that the MIOperandInfo will constrain the register class for your
16-bit offset operand to RC16_1 but in itself it won’t affect the matching of
the operand. Your SelectADDRrr will need to contain code to match an i32 added
to a sign-extended i16. If you’ve already done that, then I’m out of ideas,
sorry.

 

Steve

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